Aymenn Jawad Al-Tamimi in the Media

Syria's rebels challenge fearsome extremist group

It does not just take over villages. It repaints government buildings black, hands its fighters ministerial titles and puts them in charge of enforcing its austere vision of Islam. It speaks of Syria, Iraq and even Lebanon as one theatre of operations and boasts of its activities in all three countries.

And those who defy the Islamic State of Iraq and the Levant, known by the acronym Isis, "wind up in one of their dungeons or dead or anywhere in between", said Aaron Zelin, an expert on jihadi groups at the Washington Institute.

On Tuesday, various Syrian rebel groupings continued their days-long offensive against the group, sparked by the execution last week of a popular doctor in Aleppo province, as Iraqi troops attempted to fend off its challenge in western Anbar province.But over the last week, Isis may have over-reached itself. Just as it re-emerged in western Iraq to challenge the government of Nouri al-Maliki and claimed responsibility for its first bombing in Lebanon, the group's excesses sparked a widespread armed rebellion against it in its northern Syria stronghold.

Suddenly Isis – led by Iraqi native Ibrahim Awad al-Badri under the nom de guerre Abu Bakr al-Baghdadi – found its fearsome reputation dissipating and its weaknesses exposed as various rebel groups launched concerted attacks on its checkpoints, loosely fortified bases and even its heavily protected strongholds.

"It is spread out over too large a territory to have any kind of impregnability," said Aymenn Jawad Tamimi, a researcher at Oxford university and fellow at the Middle East Forum, which closely follows developments in Syria and Iraq. "The other rebel factions were too afraid of confronting Isis given all the trappings of the state they were setting up. They looked too powerful. It might have looked impressive, but al-Baghdadi's vision was too expansionist."

"Isis's opponents have laid their cards down on the table in terms of their real and very genuine opposition," said Charles Lister, an expert on Syrian rebel groups at the Brookings Doha Centre. "If Isis launches a counteroffensive it could have the capability to really weaken the armed opposition."The Baghdad government will probably be able to muster up enough local and international support to fend off Isis in the cities of Fallujah and Ramadi, and push the group back into the desert hinterlands of Iraq's Anbar and Nineveh provinces. But experts say it remains too early to tell how well Isis will weather its current confrontation in Syria, or what it will mean for the rebel uprising against Bashar al-Assad.

For two years Mr al-Baghdadi has proved adept at exploiting the failures of both the Syrian uprising and Mr Maliki's regime in Iraq to carve out strongholds for Isis in both countries. Weapons and young men drawn to the Syrian cause streamed back and forth across the borders and various front lines. A July prison break in Iraq replenished his forces with hardened militants.

"Without the Syrian uprising, the resurgence we see in Iraq wouldn't happen," said Valerie Szybala, who monitors Isis for the Institute for the Study of War, a Washington think-tank. "They have been bolstered by having bases in Syria, by the new flows of arms and fighters of global jihad for whom Syria is at the forefront."

From the start Isis showed it was as interested in setting up institutions with the trappings of a state – including courts, bread factories and electrical power – as in fighting Mr Assad. "When they are in control of territory, they annex it into this nebulous proto-state and make it their own," said Mr Zelin.

But according to experts the group's behaviour, rather than its ideology, turned other groups against it. For example, it rejected the arbitration mechanisms set up to resolve differences between the kaleidoscope of armed groups in Aleppo and Idlib provinces. "What they're upset about is that Isis refuses to consider itself a faction among factions," said Aron Lund, a specialist on rebel factions at the Carnegie Endowment for International Peace. "They say 'we're a state and we run our courts'. Isis never compromises and doesn't accept outside mediation."

The rebellion began after the discovery of the body of Hussein Suleiman, known as Abu Rayan, a physician and commander of the powerful the Ahrar al-Sham rebel group. Several groupings of rebels, including the recently formed Syrian Revolutionaries Front, the Mujahedin Army, and the powerful Islamic Coalition, a confederation of seven rebel groupings, joined together to attack Isis positions last Friday.

Though the slaying of Abu Rayan was seen as the catalyst for the action, Saudi Arabia, the US and others have long been pressuring other rebel groups to take on Isis, and the confrontation puts a more palatable face on the Syrian rebel movement just two weeks before a major conference on Syria in Switzerland.

"Whilst the number of opposition victories have decreased over the past year, all the ones that have succeeded have been spearheaded by Isis," said Mr Lister. "A weakened Isis will be beneficial to the regime."No one doubts that Isis, along with the al-Qaeda affiliate Jabhat Al-Nusra, has been effective in fighting the Assad regime with a ferocity that has earned it the respect of other armed groups. Jabhat Al-Nusra, which is most closely associated with Isis' hardline Islamist ideology, has largely stayed out of the battle between Isis and the other rebel groups, offering to serve as mediator and granting foreign fighters sanctuary.

Many worry the group will now deploy the same car and suicide bombs it uses against the Syrian and Iraqi regimes against the Syrian rebels. One activist who regularly communicates with Isis and follows its communications said the group is seeking revenge and seething with anger at other rebel groups, deriding them as sahwet: those fought with the US in the sahwa or awakening movement of Sunni rebels against al-Qaeda in Iraq during the last decade.

"There have been kidnaps, rapes, of wives of Isis fighters by groups like the Mujahedin Army, who attacked Isis the day after their formation," said an activist who supports Isis and requested anonymity because he is close to the group, which has been designated a terrorist organisation by the US.

He added: "As far as critical reflection [over the group's mistakes] goes, there is little."